1. Field of the Invention
The present invention relates to a method of producing a core-shell catalyst.
2. Description of Related Art
As an electrode catalyst, in particular, as an electrode catalyst for a fuel cell, a core-shell catalyst for reducing the amount of noble metal used such as platinum is known. For example, Japanese Patent Application Publication No. 2013-239331 (JP 2013-239331 A) describes a method of producing a core-shell catalyst through displacement plating to which copper underpotential deposition (Cu-UPD) is applied.
In the related art, a displacement reaction between copper and platinum on palladium-containing particle surfaces after the Cu-UPD treatment is performed using a method (hereinafter, also referred to as “dripping method”) of dripping a platinum ion-containing solution onto a copper-coated palladium-containing particle dispersion in which copper-coated palladium-containing particles accommodated in a reaction container are dispersed. The displacement reaction between copper and platinum using the dripping method is likely to occur in a state where the platinum ion-containing solution dripped onto the dispersion, in which the copper-coated palladium-containing particles are uniformly dispersed, is concentrated in the vicinity of the dripping position. As a result, a shell containing platinum is formed to be concentrated on palladium-containing particle surfaces. Therefore, in order to uniformly form a shell containing platinum on the palladium-containing particle surfaces and to improve catalytic activity of platinum per unit mass (hereinafter, also referred to as “platinum mass activity”) in the core-shell catalyst, it is necessary to slowly drip the platinum ion-containing solution while stirring the copper-coated palladium-containing particle dispersion. With the dripping method, the dripped platinum ion-containing solution cannot be made uniform in the reaction container instantly, and the platinum ion-containing solution and the copper-coated palladium-containing particles arbitrarily react with each other in a state where the concentration is not uniform. Therefore, some palladium-containing particles may be excessively or insufficiently coated with the shell, the coating state varies depending on the particles, and there is a problem in that the platinum mass activity of the core-shell catalyst is insufficient. Further, in the dripping method, in order to sufficiently advance the displacement reaction between copper and platinum on the copper-coated palladium-containing particle surfaces, it is necessary to stir the reaction solution for a long period of time after the dripping of the platinum ion-containing solution. In particular, there is a problem in that the operating cost increases during large-scale synthesis.